The subject matter of this invention relates to a method of purifying an electrolyte and an apparatus for carrying out the method.
The electrolytic deposition of metals from dissociated solutions of their salts has long been known in prior art and is used in many practical applications. In the metal solutions known as electrolytes, the salts are present in their dissociated form as ions. As a rule, electrolytes can be aqueous or organometallic systems as well as molten salts; apart from the aluminum deposition from organic electrolytes, aqueous electrolytes in particular are preferably used in electroplating and electroforming technology.
Ions are electrically charged atoms or groups of atoms which, due to their electrical charge, are able to conduct current. The electrical conductivity of the electrolytes can be further improved by the addition of acids or alkalis and/or salts thereof.
Prior to the step of the actual electrolytic metal coating procedure, it is generally necessary to subject the substrates that are to be coated to different preliminary treatments. These include, for example, degreasing, pickling, conditioning, and in the case of nonconducting substances, the deposition of conducting base layers. To carry out these preparatory steps, as a rule chemical baths are used into which the substrates to be coated are immersed. Although each of these preparatory steps is generally followed by an appropriate rinsing cycle to clean the substrate, it is not possible to completely prevent a transfer of undesirable chemicals into the electrolyte so that the electrolyte is unintentionally contaminated.
The quality of a metal film produced by electrolytic metal deposition depends decisively on the composition of the electrolyte. Thus, the goal has been to avoid a contamination of the electrolyte and thus a change in the composition of the electrolyte. However, since a transfer of the chemicals used in the previously carried out processing steps cannot be effectively avoided, the degree of contamination gradually increases over the lifetime of the electrolyte. Once a specific concentration of contaminants has been exceeded, the electrolyte is no longer serviceable and must be replaced.
One added disadvantage is that as the degree of contamination of the electrolyte increases, the probability that contaminants present in the electrolytes will be unintentionally absorbed by or incorporated into the lattice structure of the precipitating metal film, which eventually leads to the formation of defective metal films. To avoid this, it is necessary to replace a contaminated electrolyte early on with a new electrolyte which does not contain any contaminants. Against the background of environmentally benign disposal considerations, this is in most cases extremely time-consuming and, last but not least, very expensive.
An additional contamination of the electrolyte takes place during the electroless metal deposition. Thus, for example, during the ion exchange process, the ion exchange causes the nobler metal to be deposited on the less noble metal which then in turn goes into solution as an ion. In the end effect, this means that the ion concentration of the less noble metal in the electrolyte increases as the length of time during which the metal deposition process is carried out increases. Such electrolytes can be reused only to a limited extent since the serviceability of the electrolyte is compromised once a specific ion concentration has been exceeded so that the electrolyte has to be exchanged for a new one. In addition, as the ion concentration in the electrolyte increases, the insertion defect rate increases; furthermore, in the course of the deposition of the nobler metal, ions of the less noble metal can be entrained and inserted in an undesirable manner into the metal lattice structure. Thus, the following rule applies: The higher the concentration of foreign ions, the higher will be the fault insertion rate. Thus, to ensure that electroless metal deposition consistently leads to a uniform high quality, the electrolyte must be continuously monitored for the foreign ion concentration and must be replaced as soon as a predeterminable maximum concentration is exceeded. But the replacement of a contaminated electrolyte with a new electrolyte is a disadvantage not only when viewed against the background of environmentally benign disposal considerations but also because the valuable raw materials in the form of the metal ions that are dissolved in the electrolyte are wasted.
Another drawback is that electroplating baths as well as electroless baths contain inorganic and organic additives. These substances are modified and decomposed as a function of time and action (i.e., as a function of the current density, the potential or the temperature). Thus, both the quantity of the components as well as the chemical composition thereof can change. The decomposition and conversion products interfere with the electrodeposition and the electroless deposition. Therefore, these substances must be removed from the baths.